This invention relates generally to ceramic capacitors which provide DC blocking and EMI filter functions. More specifically, this invention relates to an integrated DC blocking capacitor and high frequency EMI filter in a monolithic ceramic housing.
There are two primary ceramic capacitor geometries in common use in the industryxe2x80x94the rectangular chip and the feedthrough (often called a discoidal capacitor). Ceramic capacitors are typically constructed by interleaving nonconductive layers of high dielectric constant ceramic material with metallic electrodes. The metallic electrodes are typically xe2x80x9claid-downxe2x80x9d on the green ceramic material by silk screening processes. The device is then fired (sintered) to form a rugged monolithic structure (the xe2x80x9ccapacitorxe2x80x9d). Monolithic ceramic capacitors are well known in the art for a variety of applications in both surface mount (chip capacitor) and leaded applications. Also well known in the art are stacked film capacitors, which are constructed in a very similar manner to ceramic chip capacitors. Layers of film dielectric are interleaved with conductive electrodes, thereby forming a chip-type capacitor.
The ceramic monolithic chip (MLC) capacitor (or xe2x80x9cchip capacitorxe2x80x9d) is produced in very high commercial volumes in highly automated facilities. Over the years the cost of ceramic chip capacitors has dropped a great deal. It is now common to purchase certain value chip capacitors for only a few pennies. The ceramic feedthrough capacitor is only produced in a small fraction of the chip capacitor volume. Accordingly, feedthrough capacitor production has not been nearly as automated. In addition, the feedthrough capacitor is inherently more expensive to produce due to drilling and centering the through hole, tighter dimensional control, reduced volumetric efficiency and difficulty in automating the manufacturing process. Typically the cost of a particular value chip capacitor is ten to twenty percent of the cost of an equivalent value discoidal feedthrough capacitor.
FIGS. 1 through 4 illustrate a prior art conventional MLC chip capacitor 120. The chip capacitor 120 is of standard construction, including a ceramic dielectric 122 that has disposed therein alternating lay up patterns for a first set of electrode plates 124 and a second set of electrode plates 126 separated by the ceramic dielectric 122 (FIGS. 2 through 4). The first set of electrode plates 124 terminates in a first metallization band 128 exposed at one end of the chip capacitor 120, and the second set of electrode plates 126 is conductively coupled to a second metallization band 130 disposed at an opposite end of the chip capacitor 120. The chip capacitor 120 acts as a two terminal device. That is, it is connected from one circuit trace 132 to another circuit trace 134, or from a circuit trace to ground, in order to decouple or filter signals from one line to a reference point. In the embodiment of FIGS. 1 through 4, the metallization bands 128 and 130 are soldered or otherwise conductively coupled to pads for the circuit traces 132 and 134 as shown.
FIG. 5 is an electrical schematic diagram of the chip capacitor 120 of FIGS. 1-4 illustrating its DC blocking capacitor capability. FIG. 6 is an exemplary illustration of a circuit board 136 having three of the chip capacitors 120 mounted thereon, together with other electronic components.
FIGS. 7 through 9 illustrate a prior art integrated chip capacitor 220 wherein three individual chip capacitors 120 have been incorporated into a single monolithic block 222. In the descriptions that follow, functionally equivalent elements of the various illustrated embodiments are referred to by the same reference number in increments of 100. Accordingly, the chip capacitor 220 is of a standard construction, including a ceramic dielectric 222 that has disposed therein alternating lay-up patterns of a first set of electrode plates 224 and a second set of electrode plates 226 separated by the ceramic dielectric 222 (FIGS. 8 and 9). The first set of electrode plates 224 terminates in first metallization bands 228 exposed along one side of the chip capacitor 220, and the second set of electrode plates 226 is conductively coupled to respective second metallization bands 230 disposed at an opposite side of the chip capacitor 220. Each of the chip capacitors 120 within the monolithic block 222 is connected from one respective circuit trace 232 to another respective circuit trace 234 (FIG. 10), or from a circuit trace to ground, in order to decouple or filter signals from one line to a reference point. Such monolithic ceramic chip capacitors, also known as xe2x80x9cchipsxe2x80x9d or DC blocking capacitors, are used in a myriad of applications, for example, in RF bypass, energy storage, and many other applications.
Feedthrough terminal or discoidal capacitor assemblies are generally well known for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators, or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are typically connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray electromagnetic interference (EMI) signals for transmission into the interior of the medical device. In prior devices, such as those shown in U.S. Pat. Nos. 5,333,095 and 4,424,551 (the contents of which are incorporated herein), the hermetic terminal pin sub-assembly has been combined in various ways with a ceramic feedthrough capacitor filter to decouple electromagnetic interference (EMI) signals into the housing of the medical device.
With reference to FIGS. 11-13, in a typical prior art unipolar feedthrough filter assembly (as described in U.S. Pat. No. 5,333,095), a round/discoidal (or rectangular) ceramic feedthrough filter capacitor 320 is combined with a hermetic terminal pin assembly to suppress and decouple the undesired interference or noise transmission along a terminal pin (not shown). The feedthrough capacitor 320 is coaxial, having two sets of electrode plates 338, 340 embedded in spaced relation within an insulative dielectric substrate or base 322, formed typically as a ceramic monolithic structure. One set of the electrode plates (active) 338 is electrically connected in parallel to a cylindrical metallized area at an inner diameter cylindrical surface of the coaxial capacitor structure and then to a conductive terminal pin 342 utilized to pass the desired electrical signal or signals. A second or ground set of electrode plates 340 is coupled in parallel at an outer diameter surface of the discoidal capacitor 320 to a cylindrical ferrule of conductive material (330), which is electrically connected in turn to the conductive housing of the electronic device. The number and dielectric thickness spacing of the electrode plate sets 338 and 340 varies in accordance with the capacitance value measured in microfarads or picofarads and the voltage rating of the coaxial capacitor 320. The ground electrode plates 340 are coupled in parallel together by a metallized layer 330 which is either fired, sputtered or plated onto the ceramic capacitor. The metallized band 330, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, welding, or the like. Similarly, the active electrode plates 328 are coupled in parallel together by a metallized layer 328 which is either glass fit fired or plated onto the ceramic capacitor. This metallized band 328, in turn, is mechanically and electrically coupled to the leads wire(s) by conductive adhesive, soldering or the like.
In operation, the coaxial capacitor 320 permits passage of relatively low frequency electrical signals along the terminal pin 342, while shielding and decoupling/attenuating undesired interference signals of typically high frequency (such as EMI from cellular telephones or microwave ovens) to the conductive housing.
As can be seen in FIG. 13, the feedthrough capacitor 320 is a three-terminal device as opposed to a chip capacitor, which is only a two terminal device. It is because of the nature of coaxial three-terminal capacitor devices that broad band EMI filtering is accomplished through the transmission line effect. Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quad polar (four), pentapolar (five), hexpolar (six), and additional lead configurations. The feedthrough capacitors (in both discoidal and rectangular configurations) of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a biocompatible metal such as titanium alloy which is electrically and mechanically coupled to the hermetic terminal pin assembly, which is in turn electrically coupled to the coaxial feedthrough filter capacitor 320. As a result, the filter capacitor and terminal pin assembly prevents or attenuates entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
FIGS. 14-17 illustrate a prior art flat-thru style capacitor 420. Flat-thru capacitors 420 are three-terminal devices similar to the feedthrough capacitor 320 as they are best modeled as a transmission line and also exhibit excellent high frequency performance. Flat-thru capacitors 420 are well suited for substrate or circuit board mounting, however, they are very limited in the amount of current that they can handle. This is because the circuit current must pass entirely through the electrodes themselves which are thin and lacy. In general, the flat-thru design is not used in high current applications, such as the output circuitry of an implantable defibrillator or a switch mode power supply, because of these current limitations.
The flat-through capacitor 420 includes a ceramic dielectric 422 that has disposed therein alternating lay-up patterns for an active set of electrode plates 438 and a ground set of electrode plates 440 separated by the ceramic dielectric 422. The active set of electrode plates 438 terminates in metallization bands 428a and 428b exposed at opposite ends of the flat-through capacitor 420. The ground set of electrode plates 440 is similarly conductively coupled to second metallization bands 430a and 430b disposed in a continuous band all around the opposite sides of the flat-through capacitor 420. The first metallization bands 428a and 428b are connected from one circuit trace 432 to another circuit trace 434. The circuit traces 432 and 434 comprise a circuit for conducting DC, analog, pulse or RF current. The second metallization bands 430a and 430b are connected to a circuit trace 444 which serves as a ground plane.
Operation of the flat-through capacitor 420 is similar to the discoidal capacitor 320 described previously in that the flat-through capacitor 420 permits passage of DC or relatively low frequency electrical signals along the active set of electrode plates 438, while decoupling/attenuating undesired interference signals of typically high frequency (such as EMI from cellular telephones or microwave ovens) to the conductive ground plane 444.
With reference to FIG. 17, it will be noted that the electrical schematic for the flat-through capacitor 420 of FIG. 14 is a three-terminal device which is identical to the electrical schematic for the discoidal/feedthrough capacitor 320 as illustrated in FIG. 13.
FIGS. 18-21 illustrate a prior art integrated flat-through style capacitor 520 wherein three individual flat-through capacitors 420 have been incorporated into a single monolithic block 522. In this regard, the integrated flat-through capacitor 520 is of a standard construction, including a ceramic dielectric 522 that has disposed therein alternating lay-up patterns for active set of electrode plates 538 and a ground set of electrode plates 540 separated by the ceramic dielectric 522. The active set of electrode plates 538 terminate in metallization bands 528a and 528b exposed at opposite ends of the integrated flat-through capacitor 520. The ground set of electrode plates 540 are similarly conductively coupled to second metallization bands 530a and 530b disposed on opposite sides of the flat-through capacitor 520. The first metallization bands 528a and 528b are conductively coupled from one respective circuit trace 532 to another respective circuit trace 534 which comprise individual circuits for conducting RF current. The second metallization bands 530a and 530b are connected to a circuit trace 540 which serves as a ground plane.
Operation of the integrated flat-through capacitor 520 is similar to the capacitor 420 discussed above in that the flat-through capacitor 520 permits passage of relatively low frequency electrical signals along three distinct active set of electrode plates 538, while shielding and decoupling/attenuating undesired interference signals of typically high frequency (such as EMI from cellular telephones or microwave ovens) to the conductive ground plane 540.
Since chip capacitors are used for circuit coupling and decoupling, they also provide a path where undesirable signals can be coupled. Electromagnetic interference (EMI) is a very serious concern as it can disrupt or even cause a complete malfunction of an electronic device. Chip capacitor EMI filters are relatively ineffective at high frequency in that they offer too much series inductance and therefore self resonate at a relatively low frequency.
In the past, it is common to employ monolithic ceramic feedthrough capacitors either in a rectangular or circular planar array. Where only one lead wire is involved one may use a unipolar or discoidal feedthrough capacitor. The feedthrough capacitor is a unique device in that it forms a transmission line. Accordingly, because of the geometry of the feedthrough capacitor it is effective at attenuating EMI signals over a very broad frequency range. The feedthrough capacitor does not exhibit the undesirable large value series inductance that a chip capacitor exhibits. Because of this, the feedthrough capacitor device resonates at a relatively high frequency and also continues to perform very well at above this resonant frequency. As discussed above, another form of feedthrough capacitor is known as the xe2x80x9cFlat-thruxe2x80x9d capacitor. With the xe2x80x9cFlat-thruxe2x80x9d capacitor, circuit currents pass through the capacitor itself between an opposed set of ground plates. This is also a very efficient EMI filter device, which does not series resonate in the way that a chip capacitor does.
It is common in the art to protect an electronic device or housing from the effects of electromagnetic interference by providing on each lead wire at the point of ingress or egress of the electromagnetic shield a feedthrough capacitor or a xe2x80x9cflat-thruxe2x80x9d capacitor. It is then typical to route the wires to a substrate or circuit board where DC coupling capacitors are used. This isolates the electronic device from the xe2x80x9coutside worldxe2x80x9d and provides protection in that DC bias would not be able to damage body tissue in the case of a cardiac pacemaker.
It will be appreciated, then, that there is a need for both feedthrough capacitor EMI filters and monolithic ceramic chip capacitors in selected applications. In the past, these have always been two separate and discreet components. A disadvantage of using both a feedthrough capacitor in combination with a number of discrete DC blocking capacitors is that this increases the size and the cost of an electronic assembly.
Accordingly, there is a need for a new electronic component which integrates both a feedthrough capacitor and a DC blocking capacitor into a single package. By integrated these devices into a single package, the number of components and the number of piece parts is reduced. Moreover, the volumetric efficiency is improved. The present invention fulfills this need and provides other related advantages.
The present invention comprises an integrated ceramic feedthrough filter capacitor and DC blocking capacitor in a single monolithic unit. This assembly provides for a series coupling capacitor, known as the DC blocking capacitor, which is integrated with a feedthrough capacitor or xe2x80x9cFlat-Thruxe2x80x9d capacitor. Series coupling capacitors are used in a wide variety of electronics applications to isolate one circuit from another at DC frequencies. A property of the DC coupling capacitor is that it presents a relatively low impedance at high frequencies. This makes it possible for undesirable electromagnetic interference (EMI) signals to enter into the device and cause malfunction of the electronic circuitry. The integrated device, as described herein, incorporates the properties of a DC blocking capacitor, but also includes a highly efficient high frequency EMI filter in order to protect the device from the unwanted effects of EMI. The present invention is particularly suited for use in human implantable applications, which typically employ a DC blocking capacitor on all tissue stimulation circuits. This DC blocking capacitor is mandated by the Federal Food and Drug Administration (FDA) in order to prevent damage to tissue in implantable medical devices such as cardiac pacemakers, implantable defibrillators, neurostimulators, cochlear implants and the like. All of these devices are also sensitive to EMI, such as that caused by a cellular telephone or other emitter. The present invention effectively provides DC blocking while at the same time filtering out undesirable EMI.
The integrated capacitor of the present invention comprises, generally, a monolithic casing of ceramic dielectric material, first and second sets of electrode plates disposed within the monolithic casing to form a DC blocking capacitor, and ground electrode plates disposed within the monolithic casing and between selected portions of the first and second sets of electrode plates to form an electromagnetic interference (EMI) filter. A first conductive band is provided on a surface of the casing for conductively coupling the first set of electrode plates, a second conductive band is also provided on a surface of the casing for conductively coupling the second set of electrode plates, and a third conductive band is provided on a surface of the casing for conductively coupling the ground electrode plates. These conductive bands may be disposed on either interior or exterior surfaces of the casing.
The first and second sets of electrode plates may include an induction-inducing material such as nickel to obtain desired electrical characteristics.
A discontinuous lead wire may be provided which extends at least partially into the casing. A first segment of the lead wire is conductively coupled to the first set of electrode plates, and a second segment of the lead wire is conductively coupled to the second set of electrode plates. An insulative spacer may be disposed between abutting ends of the first and second segments of the lead wire within the casing, or the segments thereof may be offset from one another. The ground electrode plates are typically conductively coupled to a conductive ferrule through which a portion of the lead wire extends in non-conductive relation.
In several of the illustrated embodiments, an integrated electromagnetic interference (EMI) filter-DC blocking capacitor is provided which includes a casing of dielectric material having generally parallel first and second sets of electrode plates disposed therein which form a plurality of distinct DC blocking capacitors, and a set of generally parallel ground electrode plates disposed within the casing between selected portions of adjacent plates of the first and second sets of electrode plates. The ground electrode plates cooperatively form, with the first and second sets of electrode plates, a EMI filter for each of the distinct DC blocking capacitors.
The aforementioned conductive bands may be provided on external surfaces of the casing for conductively coupling the ground electrode plates to the conductive ferrule, or on an internal surface. In such case, a ground pin is typically conductively coupled to the ferrule and the conductive band. The ends of the first and second segments of the lead wire may be disposed in passageways provided in the casing. The passageways may comprise through holes wherein an end of each of the first and second segments is covered with a non-conductive cap.
The ground electrode plates may be aligned with the lead wires extending into the casing or offset from the lead wires.
In alternative embodiments, the ground electrode plates may comprise a first set of ground electrode plates which are co-planar with the first set of electrode plates, and a second set of ground electrode plates which are co-planar with the second set of electrode plates. A third set of electrode plates may be provided which form an EMI filter, cooperatively with the ground electrode plates, for a lead wire extending through the casing and conductively coupled to the third set of electrode plates. Moreover, grounded shield electrode plates may be co-planarly disposed between adjacent components of the first and second sets of electrode plates to reduce cross-talk therebetween.
Other features and advantages of the present invention will become apparent from the following, more detailed description taken in conjunction with the accompanying drawings, which illustrate by way of example the principles of the invention.